1. Field of the Invention
This invention relates to high frequency signal splitter devices and more particularly to a signal splitter for CATV signal splitting.
2. Prior Art
Hybrid transformers for wideband (5-1000 MHz) high frequency applications such as Cable TV (CATV) have remained unchanged for the last twenty years. The prior art hybrid transformer design, shown in FIG. 1, consists of a 2-stage impedance transformer that converts a single input of 75 ohms into two outputs, each having a 75 ohm impedance. The device consists of a 2-stage transformer wherein the first stage converts the 75 ohm impedance to 37.5 ohms, and the second stage converts the 37.5 ohm back to two 75 ohm outputs. Since the hybrid transformer""s inception, the performance of the device as a signal splitter has been based on three factors: insertion loss, isolation between ports, and return loss or matching. These performance factors for a hybrid transformer-type signal splitter deteriorate as the total bandwidth is expanded and higher frequencies are used.
The increased use of the available bandwidth in coaxial cables for use in Cable TV has required advancement in signal splitter technology to meet new standards for performance. Twenty years ago, the splitter devices only needed to operate over a bandwidth of 54-300 MHz. Today, due to the use of over 100 analog channels, together with the addition of internet signals in the lower frequencies, signal splitters having a greatly expanded bandwidth of 5-1000 MHz are required. In addition, the minimum performance specifications for signal splitters have also increased. The increased performance standards have not been achieved by changing the circuitry of hybrid transformer signal splitter, but rather by improving ferrite core material used in the transformer and providing better grounding and layout of the device. Improved ferrite core materials have been responsible for the bandwidth and specification improvement. Specifically, the increased permeability of the ferrite core, which is the ability to contain and transfer the electromagnetic waves within the core material (efficiency), is principally responsible for the improvement in performance. Early ferrite materials used in signal splitters did not have permeabilities over 300 u needed for these new wideband applications. As the operational bandwidths reached 1000 MHz, the demand for signal splitters having high performance stimulated the development of ferrites having permeability factors of 1500 u. Such improved ferritic core materials enabled the continued use of hybrid transformer-type signal splitters meeting higher industry-imposed performance standards with the same circuit design used for 20 years.
High performance ferrites have a higher sensitivity and a lower dynamic range of signal levels which can be processed. For non-linear devices in general, and many amplifiers in particular, the desired linear performance of the device is related to the dynamic range constraints. A linear response can only be realized over a small dynamic range of input signal level. The ferrite transformer core, used in modem, high performance signal splitting devices, is a non-linear material; requiring significant limitations on the input signal level in order to operate in their linear region. In other ferrite core applications, such as magnetized data memories and RF switches, the ability to drive the material into non-linear or saturated conditions is exploited.
Cable TV applications require a signal splitter device to work linearly over a wide bandwidth but only at low input levels, usually in the range of between xe2x88x9210 dBmv (200 microvolts) to +35 dBm (80 millivolts). Internet, pay-per-view TV, and telephone signal transmission via coaxial cable require the CATV system to transmit high level return communication signals from the home at levels of 50-60 dBmv (maximum of 1 volt level). Accordingly, hybrid splitters are now required to maintain the bandwidth and performance at higher signal levels. It had been found that the second and third order non-linear distortion levels produced in response to the higher signal levels transmitted at the home are unacceptably high; the level of generated harmonics ranging from xe2x88x92100 to xe2x88x9270 dBC (from the transmitted carrier). The high level of the harmonics interfere with TV channels and other data services that fall in a 2nd or 3rd multiple of the high level signal.
Ferrite cores are magnetized by electrical spikes, noise, or surges, which are common to all communication systems connected to a power source. It has been found that when a magnetized ferrite is presented with signals at around 1 volt, as is typical with Internet applications, the harmonics also increased by a factor of 100. The above new application of high level transmitted signals through the CATV splitters has required the development of a new design which limits magnetization, has low harmonics (xe2x88x92105 dBc) at high signal input levels while maintaining the currently accepted performance levels (insertion, isolation, and return loss). While the dynamic range, magnetization resistance, and distortion can be improved with ferrite cores of lower xe2x80x9cuxe2x80x9d permeabilities, the bandwidth is unacceptable. There remains a need for a CATV splitter that can operate in a high signal level environment while maintaining high electrical performance, low distortion, and immunity from magnetization enhanced distortion.
It is a primary object of the invention to provide a CATV signal splitter having a broad bandwidth and low distortion at high signal input levels.
It is a further object of the invention to provide a broad bandwidth, high frequency signal splitter device comprising a transformer having a low permeability core material.
The features of the: invention believed to be novel are set forth with particularity in the appended claims. However the invention itself, both as to organization and method of operation, together with further objects and advantages thereof may be best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: